1. Field of the Invention
The present invention is related to a power-line carrier communication apparatus for performing a data transmission by using a power line.
2. Description of the Related Art
Power-line carrier communication apparatus own a major feature such that in-home communication networks can be immediately established by utilizing as network transmission paths, power lines which have already been installed in the respective homes. However, since these power-line carrier communication apparatus transmit/receive signals by employing such power lines having deteriorated balancing degrees as communication media, high electric power is leaked from these power lines. Also, in frequency bands required for high-speed power-line carrier communications, amateur radio communications and shortwave broadcasting programs have already utilized these frequency bands. As a result, there is a problem of interference caused by these power-line carrier communication apparatus with respect to these existing communication systems. With respect to restriction aspects ruled by radio laws and radio communication laws regulated in respective countries, various types/sorts of legal restrictions are given as to such items as usable frequency bands (ranges) and allowable electric field strengths. Accordingly, a certain limitation is necessarily required for frequency bands which are utilized for power-line carrier communications in the light of these laws. Also, since various types/sorts of electric appliances are connected to general-purpose power lines which constitute communication media of power-line carrier communication apparatus, there are many differences in impedances of power lines, noise appeared on power lines, and signal attenuation amounts during signal transmissions, which may give great influences to communication performance, depending upon wiring conditions of respective household power lines. Also, these impedances, noise, and signal attenuation amounts are changed, depending upon electric appliances connected to these power lines, and furthermore, characteristics thereof are largely varied by frequencies.
As previously explained, in such power-line carrier communications using power lines as communication media, the following risks may be conceived. That is, communication trouble and interference given to other existing communication systems may be caused by impedance changes, noise, and signal attenuation of power lines. As a result, systems capable of avoiding use of frequency bands having communication trouble should be clearly discriminated from systems flexibly adaptable to law regulations of the individual countries. In other words, frequency bands usable in communications must be clearly distinguished from frequency bands which are not in communications. Furthermore, these system changes should be readily and necessarily available. As to this problem, a large number of technical ideas using a multi-carrier transfer system have been conventionally proposed.
As a conventional power-line carrier communication apparatus in which a power line is used as a communication medium, for instance, there is such a power-line carrier communication apparatus disclosed in Japanese Laid-open Patent Application No. 2000-165304.
FIG. 25 is a block diagram for indicating the power-line carrier communication apparatus described in Japanese Laid-open Patent Application No. 2000-165304.
In FIG. 25, reference numeral 600 shows a power-line carrier communication apparatus, reference numeral 601 indicates a data divider, reference numeral 602 represents a QAM (Quadrature Amplitude Modulation) encoder, reference numeral 603 denotes an inverse Fourier transforming device, reference numeral 604 is a parallel-to-serial converter, and reference numeral 605 shows a D/A converter. Also, reference numeral 606 represents a low-pass filter, reference numeral 607 denotes a power-line coupling circuit, reference numeral 608 denotes a power line, reference numeral 609 is another low-pass filter, and reference numeral 610 indicates an A/D converter. Further, reference numeral 611 shows a serial-to-parallel converter, reference numeral 612 represents a Fourier transforming device, reference numeral 613 shows a QAM decoder, and reference numeral 614 indicates a data synthesizer.
As apparent from the apparatus arrangement of FIG. 25, in the power-line carrier communication apparatus described in Japanese Laid-open Patent Application No. 2000-165304, the orthogonal frequency division multiplexing transmission system (will be referred to as “OFDM transmission” system hereinafter) utilizing the Fourier transformation is applied to the power-line carrier communication.
Next, as to the power-line carrier communication apparatus of FIG. 25, operations thereof will now be described.
With respect to transmission operation to the power line 608, transmission data is first entered into the data divider 601 so as to produce a bit stream which is used to be allocated to a plurality of sub-carriers. Next, this bit stream is converted into complex signals by the QAM encoder 602, and then, a time sample series which has been frequency-division-multiplexed is produced by processing the complex signals via the inverse Fourier transforming device 603 and the parallel-to-serial converter 604. This time sample series is transmitted via the D/A converter 605, the low-pass filter 606, and the power-line coupling circuit 607 to the power line 608. Conversely, in reception operation from the power line 608, the A/D converter 610 converts an analog signal (power-line communication signal) into a digital signal, while this analog signal is received via the power-line coupling circuit 607 and the low-pass filter 609 from the power line 608. Next, this digital signal is converted via the serial-to-parallel converter 611 and the Fourier transforming device 612 into a QAM code with respect to each of the frequencies. Then, the respective QAM codes are demodulated by the QAM decoder 613, and these demodulated data are synthesized with each other by the data synthesizer 614.
As previously explained, in accordance with this power-line carrier communication apparatus, the transmission signal is constructed of the sub-carriers having the plural frequency spectrums by the OFDM transmission system, and the amount of information which is superimposed on these respective sub-carriers is adaptively changed in accordance with the noise of the power line and the frequency characteristic of the attenuation amount. As a result, there is such an advantage that while the frequency is utilized in a higher efficiency, the data communication can be performed by improving the transmission speed. Also, since the circuits provided on the transmission side are controlled in such a manner that an arbitrary sub-carrier is not used, such a data communication within the frequency band where the environment of the transmission path is the worst may be avoided, and since the multi-value modulation is actively carried out within the frequency band where the condition of the transmission path is better, the data communication may be carried out under stable condition. Further, under this control operation, this power-line carrier communication apparatus may output signals which are properly adapted to laws/regulations effective to the individual countries.
However, in the above-explained conventional power-line carrier communication apparatus, the below-mentioned problems occur, which will now be explained with reference to FIG. 26 and FIG. 27. FIG. 26 is a graph for graphically showing a system of a guard interval, and FIG. 27 is another graph for graphically indicating a filter band characteristic of the OFDM transmission system.
In this conventional power-line carrier communication apparatus, the OFDM transmission operation using the Fourier transformation is carried out in the data communication with employment of the power line. In this OFDM transmission operation using the Fourier transformation, such a guard interval section as shown in FIG. 26 must be provided in a signal section so as to mitigate an adverse influence by multipath aspects. In view of information transmission operation, this guard interval section becomes redundant, and therefore, reduces the frequency utilizing efficiency. The shorter the guard interval section becomes, the higher the transmission efficiency is increased. However, the adverse influence by the multipath aspects is easily given to the reception side, so that the error rate characteristic is deteriorated. Under power-line communication environment, since the delay time of the delayed wave caused by the multipath aspects is especially increased, the guard interval section must be increased. As a result, the ratio of sacrificing the transmission speed becomes extremely large. As to avoiding of the interference given to the existing system, as the conventional system, such a system has been conducted. That is, since the data is not allocated (masked) with respect to the sub-carrier, the amplitudes of the signal in the frequency band used in the existing system are theoretically reduced to zero. FIG. 19 shows an example that a frequency band which is not used in the OFDM transmission system is masked (will be explained later). Actually, amplitudes of masked sub-carriers do not appear. However, since side lobes of adjacent sub-carriers are leaked, nothing but only such an attenuation of approximately 13 dB could be obtained. In the case of the OFDM transmission system, since the Fourier transformation is carried out by using the rectangular wave as the window function, as indicated in FIG. 27, nothing but only approximately 13 dB could be obtained as to the attenuation of the side lobes with respect to the main lobe. As a consequence, the interference given to the existing communication systems cannot be sufficiently reduced. More specifically, in the frequency bands which are used in the high-speed power-line carrier communications, there are provided a large number of radio systems having higher reception sensitivities such as the amateur radio system and the shortwave broadcasting systems. To avoid the adverse influence given to these existing systems, there is such a necessity that any signals are not transmitted with respect to the frequency bands which are used by the existing systems. To this end, the band-block filter must be newly installed in the conventional method. This band-block filter may cause the circuit scale to be increased. Also, since the band-block filter must be operated in high speeds, this high-speed operation requirement may cause one of major factors for increasing power consumption.